Spark plug, and method for manufacturing the same

Abstract

A spark plug and method of manufacturing the same. The spark plug includes a tubular insulator holding a center electrode, and a tubular metallic shell holding the insulator. The insulator is inserted into the metallic shell from the rear end side thereof until a portion of the insulator engages the metallic shell. In this state, the circumferential edge of the rear end portion of the metallic shell is crimped, whereby the insulator is unitarily fixed to the metallic shell. A talc charged layer is formed in a clearance portion between the insulator and the metallic shell, and a wire packing is placed on a rear end portion of the talc charged layer for engaging the crimp portion. A portion of the talc charged layer is interposed between the wire packing and the rear trunk portion of the insulator.

1. A spark plug comprising:
 a center electrode; 
 a tubular insulator surrounding the circumference of the center electrode and having, on its outer circumferential surface, a rear trunk portion located on a rear end side thereof, and a large diameter portion located frontward of the rear trunk portion and having a diameter greater than that of the rear trunk portion; 
 a tubular metallic shell surrounding the circumference of the insulator and accommodating the large diameter portion of the insulator between a stepped portion formed inside the metallic shell and a crimp portion formed at a rear end portion of the metallic shell, to thereby hold the insulator; 
 a ground electrode provided to form a spark discharge gap in cooperation with the center electrode; and 
 an annular packing interposed between an inner circumferential surface of the crimp portion and an outer circumferential surface of the insulator, the spark plug being characterized in that 
 a cushioning member is interposed between the insulator and at least the crimp portion or the packing, whichever is closer to the insulator. 
 
     
     
       2. A spark plug according to  claim 1 , wherein the cushioning member is formed from talc powder. 
     
     
       3. A spark plug according to  claim 2 , wherein
 the talc powder is charged in a clearance portion surrounded by the inner circumferential surface of the crimp portion, a rear-end-side inner circumferential surface of the metallic shell continuous with the inner circumferential surface of the crimp portion, a rearward facing end surface of the large diameter portion of the insulator, and an outer circumferential surface of the rear trunk portion continuous with the rearward facing end surface; and 
 a radially inner portion of a rear end portion, with respect to the direction of an axis, of a layer formed of the charged talc powder projects rearward, with respect to the direction of the axis, in relation to a radially outer portion of the rear end portion, and the packing is disposed on the radially outer portion. 
 
     
     
       4. A spark plug according to  claim 1 , wherein the cushioning member is formed from a resin material. 
     
     
       5. A spark plug according to  claim 4 , wherein, as viewed in a cross section taken along the axis, the entirety of the cushioning member is located radially inward of a frontmost portion of the packing with respect to the direction of the axis. 
     
     
       6. A spark plug according to  claim 4 , wherein, as viewed in a cross section taken along the axis, the entirety of the cushioning member is located rearward of a frontmost portion of the packing with respect to the direction of the axis. 
     
     
       7. A spark plug according to  claim 4 , wherein a relative thermal index of the resin material determined in accordance with the UL-746B standard is 100° C. or higher. 
     
     
       8. A spark plug according to  claim 1 , wherein the section modulus Z of the insulator at a position facing the crimp portion is 60 or less. 
     
     
       9. A spark plug according to  claim 1 , wherein the rear trunk portion of the insulator facing the crimp portion has a diameter of 8 mm or less. 
     
     
       10. A spark plug according to  claim 1 , wherein a threaded portion which is formed on the metallic shell for attachment to an internal combustion engine has a nominal diameter of M10 or less. 
     
     
       11. A method for manufacturing a spark plug comprising:
 a center electrode; 
 a tubular insulator surrounding the circumference of the center electrode and having, on its outer circumferential surface, a rear trunk portion located on a rear end side thereof, and a large diameter portion located frontward of the rear trunk portion and having a diameter greater than that of the rear trunk portion; 
 a tubular metallic shell surrounding the circumference of the insulator and accommodating the large diameter portion of the insulator between a stepped portion formed inside the metallic shell and a crimp portion formed at a rear end portion of the metallic shell, to thereby hold the insulator; and 
 a ground electrode provided to form a spark discharge gap in cooperation with the center electrode, wherein 
 talc powder is charged in a clearance portion surrounded by an inner circumferential surface of the crimp portion, a rear-end-side inner circumferential surface of the metallic shell continuous with the inner circumferential surface of the crimp portion, a rearward facing end surface of the large diameter portion of the insulator, and an outer circumferential surface of the rear trunk portion continuous with the rearward facing end surface; and 
 an annular packing is interposed between the inner circumferential surface of the crimp portion and the outer circumferential surface of the rear trunk portion of the insulator, the method being characterized by comprising: 
 a disposing step of inserting the insulator into the metallic shell from the rear end side thereof with respect to the direction of an axis so as to dispose the insulator in a state in which the insulator is engaged with an inner circumferential portion of the metallic shell; 
 a charging step of charging the talc powder into the clearance portion; 
 a compression step of compressing a layer of the charged talc powder in the direction of the axis; 
 a placement step of placing the packing on a rear end portion of the layer of the charged talc powder with respect to the direction of the axis; and 
 a crimping step of crimping a circumferential edge of the rear end portion of the metallic shell to thereby form the crimp portion, 
 the method further comprising a placement portion forming step of forming, at least before the placement step, a placement portion, on which the packing is to be placed, on a radially outer portion of a rear end portion, with respect to the direction of the axis, of the layer of the charged talc powder.